Surgical procedures in general and spine surgical procedures in particular have progressively evolved to a minimally invasive approach over the past two decades. The advantages for the patient have been well documented with less pain, blood loss and tissue damage all contributing to a faster recovery and improved function with fewer complications. However, the smaller exposure of the surgical field has presented a challenge to the surgeon to accomplish the same goals of a successful open procedure with a technique thatless direct visualization of the operative site. To perform surgery through a smaller opening with maximum safety to the patient, newer tools and inventions have become necessary. The PediDart™ pedicle dart system contributes to the growing but still deficient list of tools required to perform safe, minimally invasive spine surgery.
Spinal fusion with internal fixation instrumentation using pedicle screws have become the “gold standard” for posterior thoracic and lumbosacral fusion using internal fixation. For optimal results and safety it is required to accurately place bone screws (pedicle screws) from the outer surface or cortex of the pedicular segment of a vertebrae and advance down the cancellous core of the pedicle to engage the threads of the screw into the body of the vertebrae. Two or more adjacent vertebra are then connected by placement of rods transfixed to the pedicle screws by a locking cap or nut, or by crimping.
To the accomplished spine surgeon such insertion of a pedicle screw is most commonly accomplished by first perforating the entry into the pedicle located at the junction of the transverse process and superior facet with a pointed metallic probe, and then following the path of least resistance through the cancellous core of the pedicle and then into the vertebral body. Proper location of the metallic probe is confirmed with x-ray or other navigational method. The path created by the probe is then tapped or threaded to prepare for placement of the pedicle screw of the proper length and dimension.
Improper or inaccurate placement of the pedicle screw can result in catastrophic injury to neurological, vascular or bowel structures or inadequate purchase of the screw to the bony vertebra resulting in unstable fixation and failure of fusion. Malplacement of pedicle screws have been reported to occur between 6-12% when evaluated by plain x-rays and up to 35% when examined with the more accurate axial computerized tomography techniques. Serious complications from malpositioned pedicle screws are approximately 2% due to the anatomic “safety margin” leaving adequate space between the vital neurologic structures and the bony spinal canal to accommodate for the error.
Nevertheless, a more exact and precise way to accurately insert a pedicle screw into the thoracic lumbosacral spine is not only desirable but necessary. Improved techniques and instruments are especially needed when “percutaneous” methods of spinal fixation are utilized such as with the “pathfinder” system (Abbot Spine), Expedium (Depuy) or Sextant (Medtronics) to name a few systems. The method of pedicle screw placement with the percutaneous systems are to first localize the entry point with fluoroscopic imaging passing a Jamshidi needle/probe through a small skin incision, docking into the pedicle entry point and then driving the Jamshidi probe through the pedicle into the vertebral body. This is followed by the insertion of a guide pin over which a cannulated tap is applied followed by pedicle screw placement. Infrared navigational methods can also be used to guide placement of the pedicle screw.
There are several drawbacks and limitations with the current technique of pedicle screw insertion. This technique has also been referred to as the “feel” technique.
(1) Entry point location by surface anatomical features are inaccurate due to bony anomalies. Variations of development, bony bumps and spurs develop differently with time.
(2) Incorrect entry points can lead to trajectory error which can result in perforation of the pedicular wall. Medial perforation will jeopardize neural structures or epidermal vessels. Lateral perforation will result in lack of stability due to failure of the screw engaging the vertebral body.
(3) The cancellous core of the pedicle may be too small (type B pedicle) or non-existent (type C pedicle) or lack continuity (type D pedicle) to allow accurate passage through the center of the pedicle to engage the vertebral body.
(4) The use of the hand-held metallic probe is physically arduous leading to surgeon fatigue.
(5) Additional steps of sequential tapping require time prior to final screw placement.
Further, the percutaneous technique has the following drawbacks.
(1) the Jamshidi probe is too flimsy and often too short, making it difficult to properly dock onto the entry point.
(2) The Jamshidi probe is too flimsy to navigate down a hard pedicle without deviating from the center of the pedicle.
(3) At times even after the vertebral body is reached, the guide wire is difficult to advance through hard bone.
(4) The additional steps of sequential tapping is required before placement of the pedicular screw.
Inaccurate placement of the pedicle screw may result in damage to the vertebrae such that screw insertion may not be possible, or if it can be achieved it may require additional surgical procedures for correction to provide the necessary holding strength.
Other conventional techniques for insertion of the pedicle screw may involve extensive paraspinous muscle dissection, impairment of surrounding tissue and other compromising tissue removal commonly necessary to properly insert a pedicle screw. This most often results in significant blood loss during this exposure phase of the operation, which is only one part of a multiple step operative procedure. Any problems at any stage of an operation can risk the patient's safety and surgical outcome.
The surgical practitioner is faced with the choice between high exposure and a good view but increased tissue damage, versus a more limited exposure with a much poorer view and a heightened risk of a wrong insertion of the pedicle screw, but with potentially better outcome.
What is needed is a system which will enable correct placement of a pedicle device, accurately and consistently and which will not require extensive paraspinous muscle dissection for proper placement. The needed device will ideally enable a gradual graded introduction into the pedicle and give an earlier indication of problems so that any error can be quickly corrected at a time before the introduction goes any further.
Currently available surgical retractor systems fail to fulfill all of the above requirements. Consequently there is a severe need for complementary tools and inventions to meet the requirements of precise, safe and timely spine fusion surgery.